In cellular communication networks, it is often desirable to determine the location of an active cellular telephone, or "mobile phone. " Applications for such a technology are numerous and include, for example, identifying the location of a mobile phone to a 911 dispatcher, providing such location information to a subscriber via a mobile phone for future data services such as computerized directions, and identifying the location of a mobile phone for law enforcement purposes.
One method of determining the location of a radio frequency (RF) transmitter, such as a mobile phone, makes use of known techniques such as time difference of arrival ("TDOA") and triangulation. For example, U.S. Pat. No. 5,508,708 to Ghosh et al. discloses a system for determining the location of an active mobile phone in a Code Division Multiple Access ("CDMA") network. In particular, using the CDMA modulation information, TDOA techniques are used to estimate the distance between a mobile phone and three base station transceiver subsystems ("BTSes"). Using triangulation, the three distances can be used to estimate the location of the mobile phone.
The foregoing system suffers from certain deficiencies, however. For example, initiation of a location determination depends on an external event trigger, e.g., a call to 911. Moreover, the system does not provide continuous tracking of the location of the mobile phone throughout a call or tracking of several mobile phones at once. Such additional capabilities could be used to simplify the handoff of a call between different BSC/MTX complexes and compiling detailed mobile traffic data for network planning, expansion, marketing studies, and other purposes. It would also be useful to be able to schedule time measurements needed for initially determining and then tracking the location of one or more mobile phones, a feature that is not provided by the system disclosed by Ghosh et al.
Therefore, what is needed is a system for locating and tracking one or more mobile phones in a cellular communication network.